1. Field of the Invention
The present invention is generally related to nuclear reactor fuel assemblies and in particular to thimble tubes in the fuel assembly guide tubes.
2. General Background
In commercial nuclear reactors, guide tubes provided in the fuel assembly have thimble tubes therein. These fuel assembly guide tubes and thimble tubes therein are spaced throughout the fuel assembly and are adapted to receive either control rods or neutron detectors. The neutron detectors are important to efficient reactor operation as they are inserted into the thimble tubes in the reactor and are used to provide flux maps of the core. The guide tubes and thimble tubes that receive the neutron detectors are part of the reactor coolant system pressure boundary and extend from a sealed table outside of the reactor vessel to the top of the core. Such instrumentation tubes normally extend through the bottom of the reactor vessel so as not to interfere with the reactor vessel cover and control rod drive mechanism. The thimble tubes are sealed at the end inside the guide tube to prevent loss of primary coolant through the thimble tube. In the reactor coolant system, coolant flows through flow holes in the lower core plate and flow holes in the lower part of the guide tubes into and through the guide tubes and exits at the top of the guide tubes. Due to their entry point into the reactor, these thimble tubes generally receive little or no lateral support along their length and are subject to flow induced vibration by the coolant during normal reactor operations. The vibrations tend to cause the thimble tubes to contact the fuel assembly guide tubes, which has resulted in through-wall wear or thinning of the thimble tubes and cracking of the fuel assembly guide tubes. Thinning of the thimble tube wall can lead to loss of integrity as a system pressure boundary and loss of reactor coolant and pressure. Cracking of the fuel assembly guide tube can lead to disruption of proper coolant flow and/or damage to the thimble tube. The addition of flow limiters has been unsuccessful as this has been reported to aggravate the problem. Plugging or repositioning the guide tubes is also not an ideal solution as this changes coolant flow or changes the positioning of neutron detectors in the core. Related patents which applicants are aware of include the following.
U.S. Pat. No. 4,229,256 discloses the use of a thimble tube having a corrugated section at its lower end to distribute the control rod deceleration forces over an extended distance in a "scram" situation.
U.S. Pat. No. 4,070,241 discloses the use of a removable radial shielding assembly for closing interassembly gaps in the reactor core assembly load plane. A flexible shielding assembly is provided with a loose fitting elongated insert in an axial opening that is bounded by upper and lower end walls. The insert is constructed of a material having a higher coefficient of thermal expansion than the shielding assembly and causes bowing thereof from pressure against the upper and lower end walls.
U.S. Pat. No. 4,318,776 discloses thimble tubes that extend through the wall of the pressure vessel of a boiling water reactor and are sealed from the interior of the pressure vessel to allow exchanging of detectors in the thimble tubes during reactor operation.
U.S. Pat. Nos. 3,664,924; 4,077,843; 4,295,935; 4,474,730; and 4,839,136 disclose the use of bimetallic spacer grids for nuclear fuel assemblies.
As it can be seen, the known art does not address the problem of damage to guide tubes and thimble tubes by flow induced vibration of thimble tubes.